Why is I2C_SMBUS_BLOCK_MAX limited to 32 bytes?
I am trying to configure a SAA6752HS chip (MPEG-2 encoder) over the I2C bus using a Raspberry Pi as my development kit. It was a piece of cake until I had to write to the 0xC2 address of the chip. For this task, I have to use an I2C command that expects a 189 byte payload. So I stumbled upon the 32 byte limit inside the I2C driver defined by I2C_SMBUS_BLOCK_MAX in / usr / include / linux / i 2c.h. It is not possible to force different values of the maximum limit. Everything around the I2C lib goes into the i2c_smbus_access function and any request with more than 32 bytes makes the ioctl return -1. I don't know how to debug it so far.
static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command,
int size, union i2c_smbus_data *data)
{
struct i2c_smbus_ioctl_data args;
args.read_write = read_write;
args.command = command;
args.size = size;
args.data = data;
return ioctl(file,I2C_SMBUS,&args);
}
I don't understand why there is such a limitation, given that there are devices that require more than 32 bytes of payload data to operate (e.g. SAA6752HS).
Is there a way to overcome this limitation without rewriting the new driver?
Thanks in advance.
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Here's the documentation for the Linux i2c interface: https://www.kernel.org/doc/Documentation/i2c/dev-interface
At its simplest level, you can use ioctl(I2C_SLAVE)
to set the slave address and system call write
to write the command. Something like:
i2c_write(int file, int address, int subaddress, int size, char *data) {
char buf[size + 1]; // note: variable length array
ioctl(file, I2C_SLAVE, address); // real code would need to check for an error
buf[0] = subaddress; // need to send everything in one call to write
memcpy(buf + 1, data, size); // so copy subaddress and data to a buffer
write(file, buf, size + 1);
}
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